Use a SourceRange for the whitespace location in FormatToken.

Replaces the use of WhitespaceStart + WhitspaceLength.
This made a bug in the formatter obvous where we would incorrectly
calculate the next column.

FIXME: There's a similar bug left regarding TokenLength. We should
probably also move to have a TokenRange instead.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182572 91177308-0d34-0410-b5e6-96231b3b80d8